1. Field of the Invention
This invention relates to a power-supply system involving system interconnection effected by a plurality of AC modules connected to a single-phase three-wire distribution line. The invention also relates to an AC module used therein.
2. Description of the Prior Art
At the present time, a power-supply system involving system interconnection is being put to practical use. As disclosed in Japanese Laid Open Patent Application No. 5-308780, this power-supply system comprises a solar battery connected to a single-phase three-wire commercial electric power system through an inverter so that electric power generated by the solar battery may be supplied to a load such as a household electric appliance.
For example, a solar battery delivers a terminal voltage of 180 V when six solar cell modules, each of which is capable of delivering a DC voltage of about 30 V, are electrically interconnected in series to form the solar battery. A DC voltage of 180 V obtained therefrom is converted by an inverter into a single-phase AC voltage of 100 V. The output terminals of the inverter are connected to two outer conductors R and T of a single-phase three-wire distribution line.
Thus, in order to apply a DC voltage of about 180 or 360 V to the inverter, the prior art power-supply system of the kind indicated above employs about six or twelve solar cell modules electrically interconnected in series, which have a disadvantage that, when a single solar cell module develops trouble, all the solar cell modules become incapable of being used.
The prior art power-supply system of the kind indicated above has another disadvantage that, in some places where the solar cell modules are installed, there is a period of time when a building or the like shades some of the solar cell modules. A decline in the output from the shaded solar cell modules results in generated energy varying with solar cell modules to such an extent that maximum output cannot be taken from the solar battery even under the control of a maximum power point tracker (MPPT).
The prior art power-supply system of the kind indicated above is provided with a protective device for cutting the connection between the inverter and the commercial electric power system when an abnormal condition occurs or an accidental power failure happens in the commercial electric power system. The protective device detects the abnormal condition or the accidental power failure, opens a switch so as to cut the connection between the inverter and the commercial electric power system within the prescribed length of time, and suspends the operation of the inverter.
In that event, therefore, electrical energy generated by the solar battery is not utilized. Such a failure to make good use of the sunlight has presented a serious problem.